Worksheet for Sizing Solar Batteries

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1. Total average amp hours per day from the System Loads Worksheet, line 10.
2. Maximum number of continuous cloudy days expected in your area.
3. Multiply line 1 by line 2.
4. Divide line 3 by (maximum) 0.8 to maintain a 20% reserve after deep discharge period. To prevent less than a maximum 80% discharge divide by a lessor number in #4 above. If no special conditions below apply, skip lines 5 through 9 and proceed to line 10.

Special Conditon #1: Heavy electrical load
5. Maximum amperage that will be drawn by the loads for 10 minutes or more.
6. Discharge rate of battery. If unknown, check with battery supplier.
7. Multiply line 5 by line 6.

Special condition #2: High Charge Current
8. Maximum output amperage of PV array or other battery charger.
9. Multiply line 8 by 10.0 hours.

10. Amp hours from line 4, 7, or 9, whichever is largest.
11. If you are using a lead-acid battery, select the multiplier below which corresponds to the battery's winter time average ambient temperature:

Battery Temperature Multiplier
80°F/26.7°C 1.00
70°F/21.2°C 1.04
60°F/15.6°C 1.11
50°F/10.0°C 1.19
40°F/4.4°C 1.30
30°F/-1.1°C 1.40
20°F/-6.7°C 1.59

12. Multiply line 11 by line 10. This is your optimum battery size in amp-hours.
13. Amp-hours of battery chosen. (Example: Concorde 1040T = 104Ah/24 hrs. and 120Ah/120 hrs. etc.)

(Note: The faster the discharge the less total reserve amp-hour capacity)

14. Divide line 12 by line 13. This is the total number of batteries in parallel required.
15. Round off to the next highest whole number.